Electrochromism is a phenomenon in which an oxidation-reduction reaction occurs reversibly and color varies reversibly by applying an electric voltage. The electrochromism is formed between two opposing electrodes, and in a configuration where an ion conductable electrolyte layer fills in between the electrodes and an oxidation-reduction reaction occurs. When a reduction reaction occurs adjacent to one of the two opposing electrodes, an oxidation reaction that is the inverse reaction occurs adjacent to the other electrode.
When a transparent display device is obtained with the electrochromic element by using the electrochromism, or by having a structure, in which three color development layers of cyan (C), magenta (M) and yellow (Y) are laminated, it is important for the element to be made of a reduced electrochromic material, a neutral state of which is a transparent state, or an oxidized electrochromic material, a neutral state of which is transparent.
Moreover, for such a brightly colored display device, an organic electrochromic material, which has various colors depending on substituent groups, is suitable. In particular, in order to enable a high contrast ratio in a transparent state (achromatic state) and a chromatic state, one of the pair of electrodes preferably has a layer including the oxidized electrochromic material and the other of the pair of electrodes preferably has a layer including the reduced electrochromic material.
However, in such a bipolar coloring electrochromic element, by repeating the cycle of coloring and decoloring, imbalance may occur between electric charges held on both poles, and electric charges remain on one of the electrodes in the achromatic state, and thereby a phenomenon of erasing residue, in which color does not completely disappear, occurs. The erasing residue is guessed to be a phenomenon occurring by a plurality of factors, such as a small amount of impurities in the device, temperature, electric voltage, or irreversible oxidation and reduction, having complex influence on each other. Furthermore, on which electrode the erasing residue occurs or how much the erasing residue is depends on usage conditions of the device. Therefore, it is difficult to completely prevent the erasing residue from occurring, or to predict it at a design phase. It is a problem.
In order to solve the problem, for example, Patent Document 1 discloses an electrochromic element in which an auxiliary electrode for charge compensation is provided and an electric voltage is applied between the electrode where an erasing residue occurs and the auxiliary electrode, and thereby the erasing residue is removed without putting an extra load on an electrochromic material for display or an electrolyte.
Moreover, Patent Document 2 discloses an electrochromic element in which an auxiliary electrode having an oxidation-reduction material is formed in a part irrelevant to the display, and an electric voltage is applied between the auxiliary electrode and an electrochromic layer, and thereby a colored state or an achromatic state can be maintained for a long period.